1. Field
The present disclosure relates to an organic light emitting diode display device. The present disclosure also relates to an active matrix organic light emitting diode display device.
2. Discussion of the Related Art
Recently, flat panel display (FPD) technology has developed to produce FPDs that are light weight and thin. Among various FPDs, organic light emitting diode (OLED) display devices are considered as a next generation display device following a liquid crystal display (LCD.
Since the OLED display devices do not need backlight (unlike LCD devices), the OLED display device has an advantage of more applications compared to the LCD device. Specifically, the OLED display device is better for transparent displays or flexible displays.
The OLED display device may be classified into two types according to an emission direction: a bottom emission type where a light is emitted toward a lower portion through a substrate having a driving circuit thereon; and a top emission type where a light is emitted toward an upper portion opposite to a substrate having a driving circuit thereon. For the purpose of obtaining a high resolution OLED display device including a flexible display and for the purpose of improving an aperture ratio and transparency of a transparent display, the top emission type OLED display device where the driving circuit does not have an influence on aperture ratio has been the recent focus.
In the OLED display device, an anode of a lower electrode functions as a pixel electrode in a pixel region and a cathode of an upper electrode functions as a common electrode over an entire surface of a substrate. As a result, light emitted from an organic material layer passes through the anode in the bottom emission type OLED display device, and a light emitted from an organic material layer passes through the cathode in the top emission type OLED display device.
Since the anode is generally formed of a transparent conductive oxide and the cathode is generally formed of a metal, the metal for the cathode is formed as a thin film to increase a transmittance in the top emission type OLED display device.
However, as a thickness of the thin film for the cathode decreases, a resistance of the cathode increases and brightness is reduced at a central portion of an image. In addition, as a thickness of the thin film for the cathode increases, a transmittance of the cathode decreases and light emission efficiency is reduced.